Two fossils unearthed in Kenya have added a new dimension to our view of life at the birth of our Homo genus. They show that two ancestral human species seem to have lived cheek-by-jowl in the same area, much as gorillas and chimpanzees do today.

Both skull fragments were found by anthropologists digging near Kenya's Lake Turkana, adding to the impressive list of early human fossils unearthed here. One of the fossils, an upper jawbone from the species Homo habilis, is dated at 1.44 million years, much younger than most fossils of this species.

The other fossil is an almost complete — but faceless — Homo erectus skull. Dated at 1.55 million years, the skull is far smaller than any other from this species — suggesting to the researchers that, as is the case with modern gorillas, there was a large size differences between the sexes in H. erectus.

Walking abreast

The fact that these two species seem to have been contemporaries is a surprise to anthropologists, say Fred Spoor of University College London and his colleagues, who discovered the hominin fossils seven years ago and now describe them in this week's Nature1.

Anthropologists have tended to see the evolution of Homo species as a linear progression, beginning with H. habilis and passing through H. erectus before ending up with modern humans. But it seems the path through time was broad enough for more than one species to walk abreast, with H. erectus and H. habilis living in the same place at the same time for as much as half a million years. Spoor and his colleagues argue that this makes it less likely that H. erectus was a direct descendant of H. habilis, instead suggesting that there is a common ancestor yet to find.

The two species are thought to have lived side by side in much the same way as modern chimps and gorillas coexist in central regions of Africa — by adopting different habits and diets. "To live in the same area for half a million years they must have found their own niches — different diets, maybe different migratory routes — to minimize competition," says Spoor. "When food is scarce, when there's a drought or something, it becomes very important that you're not in each other's way."

Harem of females

The new H. erectus skull also changes our ideas about the nature of this species. "What is truly striking about this fossil is its size," comments Spoor. The fact that the skull — probably belong to a young adult — is so small suggests that the size range of H. erectus was much larger than we imagined. The researchers infer from this that the males of H. erectus were much bigger than the females. By comparison, there is a relatively slight difference seen between the sexes in our own species. A greater inequality of size has implications for the way the creatures lived.

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H. erectus has always been viewed as similar to H. sapiens in both body shape and lifestyle. Spoor points out that the new discovery suggests a family set-up more akin to that of modern gorillas in which dominant males mate with a harem of females. "If we look at those primate species that have large sexual dimorphism, their groups usually involve one dominant male — the silverback if you're talking about gorillas — multiple female mates, and then perhaps a few non-dominant males that hang around, just waiting for their chance," Spoor says.

A similar set up is inferred from fossils of the earliest hominins, such as the australopithecines, but there has been a widespread assumption that sexes of more or less equal sizes arose when our ancestors ditched their more ape-like characteristics, evolving from Australopithecus into the more genteel Homo. To find such a difference in H. erectus, Spoor says, "was quite a surprise, actually".